Analysis of Two-Unicast Network-Coded Hybrid-ARQ With Unreliable Feedback

Network-coded (NC) automatic repeat request (ARQ) systems have been shown to provide significant throughput improvements over basic ARQ systems in multiuser wireless systems. These systems have been extended to NC hybrid-ARQ (HARQ) to show further enhancements in throughput by allowing receivers to use previous retransmissions. However, prior results were derived under the assumption of perfect ARQ feedback channels. In practical systems, feedback channels are not perfect, and therefore it is crucial to study the effect of noise in the ARQ feedback channels. In this paper, we consider the case of two-unicast flows and extend NC-HARQ to the case of imperfect ARQ feedback. Assuming binary symmetric feedback channels, we analyze the performance of these systems in terms of throughput, probability of packet loss, and average number of retransmissions. In the presence of feedback noise, our results show that significant throughput improvements over basic HARQ can still be obtained for a moderate-to-high signal-to-noise ratio with negligible increase in the probability of packet loss. We also present a new scheme that limits the number of NC packets to only one per data packet and show that for the two-unicast case the new scheme attains mast of the throughput improvements of the original NC-HARQ scheme.